In a distribution box of the above type, the bent feed duct is arranged for connection to the outlet of a motorised fan or blower unit, for example comprising a turbine or a blower for forcing fresh air from outside the passenger compartment of the vehicle into the distribution box; alternatively this air may be drawn from an air conditioning unit. The fresh air propelled by the motorised unit first flows into the feed duct and then into the air inlet duct or ducts, after which it passes through the distribution box in which it may, if required, be heated by means of a heat exchanger. Finally the air is blown into the passenger compartment through the outlet ducts.
In arrangements of this type, the distribution of flow velocities and mass flows of the air is not homogeneous at the outlet of the turbine or blower of the motorised unit, and this is therefore also true at the inlet section of the air inlet duct or ducts. It follows that it remains non-homogeneous downstream of the latter, in the heating and/or air conditioning means itself. In order to overcome this disadvantage it has previously been proposed to provide deflector vanes mounted in the bent feed duct and alinged towards the input section of the input duct or ducts through which air flows into the distribution box, in such a way as to try and obtain as homogeneous a distribution as poossible for the flow velocities and mass flows of the air passing into the inlet duct.
This known solution does however also have certain disadvantages. First of all this homogeneous distribution has proved difficult to achieve in practice, and calls for each feed duct to be manufactrued in a particular special way. Also, this result can in any case only be obtained over a rather limited range of operating speeds of the turbine or blower. If these speeds are increased above this range, the air streams become detached from the deflector vanes, so that certain zones of the inlet duct or ducts then cease to receive any air.
In addition it is known from the French Patent Application No. 86 08115, publication No. 2 583 000 (U.S. Pat. No. 4,712,611), to provide a baffle which is of generally grid-like form to impart some homogeneity to the air stream passing through the heat exchanger of a heating or ventilating apparatus. However, this calls for an apparatus of different construction, since the feed duct which supplies fresh air to the input ducts is not bent. In addition, this known solution has the major disadvantage of achieving a homogeneous distribution only at the cost of a substantial loss of pressure due to the presence of the baffle.